The technique for producing polyurethane foams has been long known. It consists in reacting polyether-polyols with polyisocyanates in the presence of catalysts, of foaming agents, of cell-regulator surfactants, of stabilizers and of further auxiliary products.
The polyether-polyols presently used to produce flexible polyurethane foams are prepared by condensing one or more alkylene oxides on compounds containing at least two activated hydrogen atoms, such as, e.g., glycols, triols, tetrols, amines and their mixtures.
The more commonly used alkylene oxides are ethylene and propylene oxides, used either alone or as mixtures. For a mixture of propylene and ethylene oxides, the ethylene oxide content of such mixtures should not exceed 45% by weight, in that both the polyol and the resulting polymer would show a too high surface tension. This would prevent the polyurethane from regularly growing in the presence of the silicone surfactants used at present.
The use of polyethers with a high content of ethylene oxide is anyway necessary if one wants to produce flexible, high-softness polyurethane foams.
To overcome these drawbacks, French patent application No. 2,129,823 proposes to improve the processability of the polyol with a high content of ethylene oxide (58-77%) by mixing it with another polyol, with a low content of ethylene oxide (4-15%).
In such a way, the resulting system has a low enough value of surface tension to be suitable for being processed with the customary silicone surfactants.
A disadvantage which affects the process according to the above cited French patent is that the use is required of two polyols which are not mixable with each other, and furthermore that polyols are used which contain a large number of primary hydroxy radicals, which require, in order to yield the end polymer, one single catalyst (i.e., an aminic catalyst).
As a consequence thereof, the suitable balancing between the reactions of polymerization (NCO and OH) and of foaming (NCO and H.sub.2 O) cannot be obtained; such balancing is normally carried out, in the continuous-block technology of flexible foams, by a pair of catalysts, i.e., a metal-containing catalyst [tin-(II) octanoate, dibutyltin dilaurate, and so forth] and an aminic catalyst (tertiary amines).
In Italian patent 1,858,454 a process is disclosed according to which only one single free polyether-polyol is used. Therefore, such a process does not suffer from the above said drawbacks in the production of flexible polyurethane foams.
In particular, in said Italian patent a process is disclosed for preparing flexible polyurethane foams having an extremely high softness, according to which a polyisocyanate is reacted with a hydroxy-compound in the present of catalysts and of foaming agents and of still further additives, as generally used in the production of polyurethane foams, in which the hydroxy-compound used is a polyether-polyol containing from 75 to 90% by weight of ethylene oxide, and displaying the following characteristics:
(a) a functionality equal to, or higher than, 2, and preferably within the range of from 3 to 4; PA0 (b) an equivalent weight within the range of from 700 to 2200 per each hydroxy radical; PA0 (c) a content of primary hydroxy radicals, relatively to the total hydroxy radicals, within the range of from 2 to 35%; and finally PA0 (d) a ratio of the content of ethylene oxide to the primary hydroxy groups within the range of from 2.1 to 42.5; PA0 (a) a hydroxy functionality within the range of from 2 to 4; PA0 (b) an equivalent weight within the range of from 700 to 2500 per each hydroxy group in chain-end position; and PA0 (c) a content of primary hydroxy groups in chain-end position, relatively to the total amount of hydroxy groups, within the range of from 0 to 100%, and preferably within the range of from 36 to 100%. PA0 (a) a polyisocyanate of general formula (I) or (II) is reacted with at least one polyether-polyol having a molecular weight within the range of from 1000 to 8000, and preferably within the range of from 3000 to 6000, having a hydroxy functionality within the range of from 2 to 4, such that a modified polyisocyanate is obtained, which contains an amount of free NCI radicals which is within the range of from 10 to 45% by weight, and is preferably within the range of from 15 to 40% by weight; PA0 the so obtained modified polyisocyanate is reacted with a compound containing activated hydrogen atoms in chain-end position, comprising at least one polyether-polyol obtained by the condensation of propylene oxide and ethylene oxide, with a content of ethylene oxide within the range of from 10 to 90% by weight, and preferably within the range of from 30 to 74% by weight, and having:
and the polyisocyanate used is the product from the partial polymerization of toluene-diisocyanate with polyether-polyols, with said polyisocyanate and said hydroxy-compound--to which foaming agents, and so forth, are added--being used in such amounts that the ratio of NCO/OH in the reaction mixture is higher than, or equal to 1, and is preferably within the range of from 1 to 1.15.
Unfortunately, by the process disclosed in Italian patent No. 1,858,454, only polyurethane foams can be obtained whose load-bearing capacity, as measured according to ISO Standard 2439 under a compression rate of 40%, is very low, and typically is lower than 50 N, within the range of bulk densities which is customarily adopted in this application sector (i.e., of from 15 to 60 kg/cm.sup.3), in that the formulation taken into consideration, when used in combination with the customarily used foaming agents--prevailingly based on chloro-fluoro-alkanes--do not make possible higher values of load-bearing capacity to be achieved.